She was appointed lady-in-waiting to Hedwig Elizabeth Charlotte of Holstein-Gottorp and became a center in the social circles of the court, where she was admired for her musicality (she was a good Harpplayer), and often played the main part in the amateur theatre at court. She was pointed out and known as the mistress of King Charles XIII after his accession to the throne in 1809, and eventually as the mistress also to his successor King Charles Bernadotte (crowned 1818), with whom she was said to have had a child. Historians have sometimes been uncertain, whether she actually functioned as a mistress, or only seemed to have this position. Queen Hedvig Elizabeth Charlotte remarked that the king acted as her lover without being able to, while Bernadotte became involved with her after his wife left for France (1811). Bernadotte tried to conceal the affair, while Koskull did the opposite: In June 1815, the queen remarked in her famous diaries that Koskull exposed new jewels so expensive that the were an obvious gift from the Prince. Koskull made recommendations and had her brother Gustav made captain lieuntenant for the royal guard. A painting of her as the muse of music was placed in one of the salons of the king. In the back of the painting, another painting was placed with the image of a fortune teller. When the painting was turned with the image of the fortune teller visible, it was a sign to Bernadotte that that day was her day with the king: if it was turned showing the image of the muse, then she would spend the day with Bernadotte.